My script is currently monitoring the hereNow value for a couple of venues. When working with this data, though, I do not want to count the same checkins twice; when a user checks into a venue, after how long does this checkin expire?
The hereNow count reflects the number of users currently checked in at a venue. Polling the hereNow count of a venue to determine the number of unique users who check in at a venue is not recommended, and will not work reliably. If you are the manager of the venue, you can use the venue manager API, or the venue push API to get reliable data about how many users have checked in at the venue.